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CHAPTER VII. — PHENOMENA OF VEGETATION. - PARASITES. 391 



localisation. Other species partly belonging to the same genera show the opposite 

 behaviour. The mycelium of Puccinia Tragopogonis which bears aecidia for instance, 

 spreads all through the host and produces its aecidia over the entire surface of the leaves, 

 while that which grows from the aecidiospores and forms teleutospores is confined 

 to small spots on the leaf. The same is the case also with Uromyces Pisi, the con- 

 verse with Melampsora Goppertiana. 



The varying extent to which Tuburcinia Trientalis spreads (see page 180) 

 according to the kind of spore from which it grows is a nearly allied phe- 

 nomenon. 



But mycelia grown from similar spores appear in some cases to be narrowly 

 localised or to spread widely according to the species of the host. Cystopus candidus 

 for example often appears to be limited to narrow spots in the leaves of species of 

 Brassica, into which it must have penetrated through the stomata of the full-grown 

 leaves, while it behaves in exactly the opposite way in Capsella and Lepidium. Its 

 mode of entrance in Brassica would be in accordance with experience, for I once 

 observed that the germ-tubes of this Fungus were able to develope a mycelium in the 

 full grown leaves of Heliophila crithmifolia. It is not improbable that some Uredineae 

 are affected in the same way by a difference in their hosts, but the point requires 

 further examination. 



In many Fungi the mycelia which spread through the host have everywhere 

 the same characters, or at least they have no distinct and special characters at distinct 

 places in the host ; they may form their spores at any spots where the external 

 conditions, as supply of air or peculiar mechanical relations, permit, and actually do 

 so form them. 



On the other hand there are many species which assume different characters 

 in different organs of the host, showing structural differences in the hyphae themselves, 

 and especially capacity or incapacity for forming sporocarps and spores. It is obvious 

 that conditions of nutrition must in general be the cause of this phenomenon, but we 

 have at present no precise physiological account of the matter. We can only therefore 

 speak provisionally in such cases of favourite places for the formation of one and 

 another kind of spore. Some remarkable examples have been already briefly given 

 in Chapter V, but we may add a few more in this place. 



Cystopus Bliti forms its gonidia only on the leaves of Amarantus Bliti, and its 

 oospores only in the stems ; Cystopus candidus forms gonidia in great abundance on 

 all the aerial organs of its hosts, but I never found its oospores on the leaves ; some 

 species of Peronospora behave in the same way ; P. Arenariae, Brk., for example, 

 produces gonidia on all parts of the leaves of Mohringia trinervia, oospores almost 

 exclusively in the parts of the flowers. Very many Ustilagineae form their spores only 

 in or on the parts of the flowers of the host, some in the anthers, others in the ovary 

 or ovule, others again (Sorosporium Saponariae, Ustilago Tragopogonis) on the whole 

 surface of the corolla and of the parts inclosed by it. It has already been remarked 

 that Peronospora Radii and P. violacea are also confined to the flowers of the host for 

 the formation both of oospores and gonidia. In other Ustilagineae, as Urocystis 

 occulta, certain parts of the leaves are the places where the spores are formed, in 

 4 Ustilago hypod/tes chiefly the surface of the stem where it is covered by the leaf- 

 sheath. Epichloe typhina always forms its stromata on the outer surface of the sheath 



